Flame Emission Spectra

As can be seen in Fig. 5.44 as well, flames in gas heaters have a similar emission spectrum. Besides the UV surveillance ionization electrodes are often used in gas burners. The method is cheap and secure but it disturbs the combustion process since the electrode has to be placed close to the flames. New developments in gas heaters focus on catalytic combustion on a metal mesh. There, an ionization electrode would fail due to the lack of a flame. However, the characteristic UV emission is still present... 

In power stations or other combustion units above a capacity of, say, 1 MW, UV sensors for flame monitoring are not unusual. Here, even combustion parameters like the air or fuel supply are controlled by sensing the UV emission spectrum of the flames. 

The chemiluminescent emission spectrum of GeCl2 was obtained by burning GeCl4 in potassium vapor using a diffusion flame technique 11 The spectrum consisted of a series of closely spaced diffuse bands in the region 4900—4100 A with an underlying continuum. The bands resemble those of SnCl2. 

The resulting neutral atoms are excited by the thermal energy of the flame which are fairly unstable, and hence instantly emit photons and eventually return to the ground state (i.e., the lower energy state). The resulting emission spectrum caused by the emitted photons and its subsequent measurement forms the fundamental basis of FES. 

How is the atomic emission spectrum of an element related to these flame tests ... 

A convenient method is the spectrometric determination of Li in aqueous solution by atomic absorption spectrometry (AAS), using an acetylene flame—the most common technique for this analyte. The instrument has an emission lamp containing Li, and one of the spectral lines of the emission spectrum is chosen, according to the concentration of the sample, as shown in Table 2. The solution is fed by a nebuhzer into the flame and the absorption caused by the Li atoms in the sample is recorded and converted to a concentration aided by a calibration standard. Possible interference can be expected from alkali metal atoms, for example, airborne trace impurities, that ionize in the flame. These effects are canceled by adding 2000 mg of K per hter of sample matrix. The method covers a wide range of concentrations, from trace analysis at about 20 xg L to brines at about 32 g L as summarized in Table 2. Organic samples have to be mineralized and the inorganic residue dissolved in water. The AAS method for determination of Li in biomedical applications has been reviewed . 

Yellow flame color is achieved by atomic emission from sodium. The emission intensity at 589 nanometers increases as the reaction temperature is raised there is no molecular emitting species here to decompose. Ionization of sodium atoms to sodium ions will occur at very high temperatures, however, so even here there is an upper limit of temperature that must be avoided for maximum color quality. The emission spectrum of a yellow flare is shown in Figure 7.2. 

T. Melville noted in 1752 that sodium colours the flame of alcohol yellow, and A. S. Marggraf used this as a test to distinguish sodium from potassium salts. With an ordinary one-prism spectroscope, the spectrum appears with a single yellow fine corresponding with the D-line of the solar spectrum. This line really consists of two lines of wave-length 5896 and 5890. The emission spectrum of sodium shows many other lines of feeble intensity. In a salted Bunsen s flame, practically... 

The laser atomic fluorescence excitation and emission spectra of sodium in an air-acetylene flame are shown below. In the excitation spectrum, the laser (bandwidth = 0.03 nm) was scanned through various wavelengths while the detector monochromator (bandwidth = 1.6 nm) was held fixed near 589 nm. In the emission spectrum, the laser was fixed at 589.0 nm, and the detector monochromator wavelength was varied. Explain why the emission spectrum gives one broad band, whereas the excitation spectrum gives two sharp lines. How can the excitation linewidths be much narrower than the detector monochromator bandwidth ... 

Fluorescence excitation and emission spectra of the two sodium D lines in an air-acetylene flame, (a) In the excitation spectrum, the laser was scanned, (to) In the emission spectrum, the monochromator was scanned. The monochromator slit width was the same for both spectra. [From s. J. Weeks, H. Haraguchl, and J. D. Wlnefordner, Improvement of Detection Limits in Laser-Excited Atomic Fluorescence Flame Spectrometry," Anal. Chem. 1976t 50,360.]... 

The ground state C1(Xl L ) is a primary product of acetylene photolysis. The r/ ll state is formed from the photolysis of bromoacetylene in the vacuum ultraviolet. It is also formed in flame and discharges through carbon containing compounds. The Swan system is a major feature of emission spectrum from the heads of comets. 

A. Schuster 8 found that the spectrum of ammonia in the discharge tube shows a broad, greenish-yellow band between 5688 and 5627. G. Magnanini observed the spectrum of the flame of ammonia burning in oxygen exhibits a large number of hydrogen lines. This spectrum was also observed by J. M. Eder, who measured 240 lines between A=5000 and 2262 for the extreme ultra-violet. The emission spectrum has seven characteristic bands—one between the red and ultra-violet. 

Sodium salts, when heated in a flame, give that flame a bright yellow color, and this color matches the two brightest lines in the emission spectrum of sodium. Sodium is found widely in nature, and lots of substances produce these lines. Fraunhofer had found that the spectrum of a candle flame contained two bright lines precisely corresponding to two dark lines, known as the D lines, in the emission spectrum of the sun. 

Figure 8 Part of the emission spectrum from an air-acetylene flame into which a...

The photomultiplier detects both the thermal emission from the determinant and also any other atomic or molecular emission from either concomitant elements present in the sample or from the flame itself. Figure 8, for example, shows a typical section of a flame emission spectrum. While it is possible for some determinations by FES to work at a single fixed wavelength, as in flame AAS, it is advisable, at least initially, to scan the emission spectrum in the vicinity of the wavelength of interest to confirm the absence of spectral interferences. In any event, regular re-zeroing and aspiration of an appropriate standard to check for signal drift is essential. 

Not all transitions which are observed in the emission spectrum have the unexcited state (the ground state) as their lower energy level. In other words they require partial excitation before atomic absorption can occur. However, in flames, most atoms exist only in the ground state, and only transitions with the ground state as their lower energy state exhibit sensitive absorption.13,14 Because the number of such transitions is small, the probability of overlap of the atomic absorption line profile of one element with the emission line profile of another element is extremely small. The spectral selectivity of AAS is therefore excellent in this respect. 

In AFS there is no need to isolate a single wavelength in the fluorescence emission spectrum from nearby, less-intense emission wavelengths, since all lines contribute to the fluorescence signal. Therefore quite large spectral bandpasses are often employed in flame AFS, especially when a low-background flame is being used. Indeed, as seen in Chapter 2, section 14, non-dispersive, filter-based systems may sometimes be employed.7,8... 

If the flame background around the wavelength to be used is variable, as a consequence of variable amounts of potentially interfering matrix elements, it is advisable to scan the emission spectrum in the vicinity of each wavelength of interest for each sample and standard in routine analysis. Spectral scan rate, the wavelength interval studied for each determination, and electronic damping must all then be carefully optimized. 

Silicate, nickel, and cobalt tend to interfere in the air-acetylene flame, although nickel and cobalt are rarely present in sufficient excess to cause a problem. Silicate interference may be eliminated at modest excesses by the use of lanthanum as a releasing agent or by using a nitrous oxide-acetylene flame. Very careful optimization is sometimes necessary, for example in the analysis of freshwaters, when concentrations are very low. It is important to use a narrow spectral bandpass and to make sure that the correct line is being used, because the hollow cathode lamp emission spectrum of iron is extremely complex. If you have any doubts about monochromator calibration, check the sensitivity at adjacent lines ... 

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